RELATED RESOUCES:
In the analysis of unsignalized intersections, it is important to know the minimum size of the headway in traffic flow that drivers are willing to accept when entering or crossing a traffic stream. The length of a headway is measured from the front of one vehicle to the bumper of the next. Although many methods exist to compute the expected number of headways of a given size in the traffic stream, the traffic engineer usually must assume the minimum headway drivers actually will use. Based on this assumption, there are methods to identify the level of service at an intersection, as well as the capacity of each of its approaches. For instance, the 2010 Highway Capacity Manual’s analysis methodology assumes a minimum acceptable headway in the flow of conflicting traffic through an intersection without signals, then computes the level of service and capacity for each of the movements. Pedestrians preparing to cross the roadway must assess the headways in the conflicting traffic, determine when one of sufficient length exists, and decide if they will cross. Again, there is a methodology that allows the investigator to compute the number and size of the headway expected and identifies whether there are sufficient headways to allow safe pedestrian passage. However, all of the computation methods require identification of the minimum headway before calculations can be made.
Equipment The equipment list for data collection procedure is a stopwatch (cell phone) and note-taking materials.
Visit the site, and choose a location and time to collect the data For this lab session, the student groups will gather data to identify the minimum acceptable headway for left-turn and right-turn traffic from the minor roadway using simulation video. The major roadway should have relatively high volume.
Collect and record the data A time headway is measured from the time the last conflicting vehicle passes the intersection until the next conflicting vehicle passes it. A useable headway is one that a minor-road vehicle uses to execute a maneuver. Use the attached form to record the useable headways (in seconds) in the conflicting traffic. For the right-turn maneuver, the conflicting traffic consists of only the vehicles traveling in the near lanes of the major highway—those lanes in which the right-turning vehicle will merge. For instance, if the minor traffic approaches the intersection northbound and wishes to travel eastbound (turn right) on the major roadway, then only the eastbound traffic conflicts with the movement. Likewise, should the vehicle wish to travel westbound (turn left) on the major road, then both eastbound and westbound traffic are conflicting. For each minor-approach vehicle, the headway that is used is measured and the time is recorded. Should a vehicle arrive at the intersection and not have to wait for a conflicting vehicle to pass before proceeding, then the time is measured from when the vehicle moves into the intersection until the next conflicting vehicle arrives. Should multiple vehicles use the headway, only the first vehicle is recorded as passing and the size of the headway is measured. At least 50 left turns and 50 right turns from the minor roadway should be observed.
Check your work before leaving the field Before leaving the field, the data collection form should be checked to see that all field data have been collected correctly. Any unusual observations should be listed as comments.
Summarize field data on the provided form To properly arrive at a minimum useable headway, the collected observations need to be summarized. The Headway Study Data Summary Form attached to this handout allows for the headways to be grouped in 0.25-seconds increments. For the first increment (3.0 entry in the “headway” column), the number of headways measuring 3.0 to 3.25, inclusive, is entered in the “# Observed” column. The frequency is computed by dividing the entry by the total number of observations. The cumulative frequency is computed by summing all frequencies up to, and including, the row being calculated. The calculation is similar to that performed in Session 1 (Spot Speed Study).
Identify minimum acceptable headway Identification of the minimum acceptable headway from the observations requires a criterion. In traffic engineering, a criterion is usually expressed in percent of drivers or vehicles for which a design standard applies. This recognizes that traffic engineers cannot design for every vehicle-driver combination. For this lab session, an 85% acceptance rate will be the criterion. Therefore, the student wishes to identify the smallest headway that 85% or more of the drivers observed would use. This value is identified by scanning down the “Cumulative Frequency” column until the first entry is found that is ≥0.15. The maximum headway size for that row is the minimum headway that 85% of the observed drivers need in order to execute that maneuver. For instance, if the 5.25-second row has a cumulative frequency of 0.16 for the left-turn move, then the minimum acceptable headways can be said to be 5.50 seconds—the upper limit of the group. That means 84% of the drivers will use a headway at least 5.50 seconds in length.
Compare observed values to standardized values The student should compare the observed minimum headways to the standard values used in Highway Capacity Manual (HCM) analyses. The HCM uses the minimum headways shown in the following
| Veh. Maneuver | 2-lane Major Road | 2-lane Major Road |
|---|---|---|
| Right turn from minor street | 6.2 s | 6.9 s |
| Left turn from minor street | 7.1 s | 7.5 s |
\(Pr(h≥t)=e^(-qt/3600)\)
where q= flow in vehicles per hour. Given a minimum acceptable headway of 5.50 seconds, the probability of headways greater than, or equal to, this minimum value on a roadway with 300 vehicles per hour is 0.632 or 63.2%. Additionally, the expected number of acceptable headways that may be expected can be calculated by multiplying the vehicle volume, minus one, by the probability. For the calculation completed:
\(Frequency=(q-1)× e^{-qt/3600} =(300-1)×0.632 = 189 "headways\ per\ hour\)
This indicates that sufficient headways in the traffic exist at this intersection for 189 vehicles per hour to be accommodated by the minor approach.
Input data for the report: Assume the flow of traffic, \(q = 450\ vehicles\ per\ hour.\)